We report direct measurement of surface deformation in soft solids due to their surface tension. Gel replicas of poly(dimethysiloxane) masters with rippled surfaces are found to have amplitudes that decrease with decreasing gel modulus. Surface undulations of a thin elastomeric film are attenuated when it is oxidized by brief exposure to oxygen plasma. Surface deformation in both cases is modeled successfully as driven by surface tension and resisted by elasticity. Our results show that surface tension of soft solids drives significant deformation, and that the latter can be used to determine the former.

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We measured the shape change of periodic ridge surface profiles in gelatin organogels resulting from deformation driven by their solid-vapor surface stress. A gelatin organogel was molded onto poly-dimethylsiloxane (PDMS) masters having ridge heights of 1.7 and 2.7 μm and several periodicities. Gel replicas were found to have a shape deformed significantly compared to their PDMS master. Systematically larger deformations in gels were measured for lower elastic moduli. Measuring the elastic modulus independently, we estimate a surface stress of 107 ± 7 mN m(-1) for the organogels in solvent composed of 70 wt% glycerol and 30 wt% water. Shape changes are in agreement with a small strain linear elastic theory. We also measured the deformation of deeper ridges (with height 13 μm), and analysed the resulting large surface strains using finite element analysis.

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Nanoscale self-assembly is a concept that Nature has been making use of since the beginning of life, [1] and we have only recently started realizing its potential for achieving better control over materials properties. An overwhelming number of self-assembly fabrication methods for the formation of nanocluster arrangements on surfaces have been found, and they feature processes having various timescales, complexities, and versatilities. The fabrication of such structures can be discussed in terms of two processing steps that are independent of the actual process sequence. First, nanocomponents must be formed, which can be accomplished in various ways from precursors in the liquid, solid, or gas phases employing either chemical or physical deposition processes. In the second step, the challenge is to organize the segregated and deposited nanoparticles into structures or patterns on surfaces. Several approaches that use either a serial-writing-type process [2] or templates that allow fast parallel processing [3,4] have been formulated. More simple are the template-free approaches based on self-organization, which is the autonomous organization of components into patterns without human intervention.[5]Among all of these approaches, wet-chemical strategies utilizing fluid mechanics appear to be the simplest and most effective. Evaporation of drops on nonfunctionalized substrates has been used for the patterned deposition of solutes in DNA microarrays.[6] Furthermore, the ring deposition of particles from colloidal dispersions deposited as droplets has also been shown. [7] In addition to droplet drying, combined flow drying has been utilized for the deposition of semiconductor nanowires and nanotubes onto functionalized substrates.[8] The self-organization of matter into regular sub-micrometer-and nanoscale lines by using the wetting instability of a Langmuir-Blodgett film [9,10] as a cost-effective method has also been discussed. However, structuring of nanocluster arrays or wirelike morphologies from a droplet still faces certain challenges. Yawahare et al. [11] have demonstrated the room-temperature formation of lines from a drop if three prerequisites are met: evaporation between partially wetted surfaces, the presence of a pinning point, and the availability of a surfactant. In contrast to the above approaches, this paper presents a droplet-deposition-based, template-free, and rapid (only a few seconds) approach for fabricating nanostructures without the use of any surfactant. Our general setup can be understood as the so-called "anti-Lotus effect". The Lotus effect [12] is well known for its removal of dust particles from the surface of a lotus leaf by gathering them into a droplet that is moving over the surface, thus cleaning it. The effect is based on the ability of certain surfaces to form spherical droplets with contact angles near 180°(i.e., superhydrophobic), enabling the incorporation of surface particles as well as a reduction in friction. In contrast to this, our work makes use of an anti-Lotus effect,...

We tested the adhesive response of polymer surfaces structured with arrays of cylindrical fibrils having diameters of 10 -20 mm and aspect ratios 1-2.4. Fibrils had two different tip shapes of end-flaps and round edges. A preloadinduced mechanical buckling instability of the fibrils was used to switch between the states of adhesion and non-adhesion. Non-adhesion in fibrils with round edges was reached at preloads that caused fibril buckling, whereas fibrils with end-flaps showed adhesion loss only at very high preloads. The round edge acted as a circumferential flaw prohibiting smooth tip contact recovery leading to an adhesion loss. In situ observations showed that, after reversal of buckling, the end-flaps unfold and re-form contact under prevailing compressive stress, retaining adhesion in spite of buckling. At very high preloads, however, end-flaps are unable to re-form contact resulting in adhesion loss. Additionally, the end-flaps showed varying contact adaptability as a function of the fibril -probe alignment, which further affects the preload for adhesion loss. The combined influence of preload, tip shape and alignment on adhesion can be used to switch adhesion in bioinspired fibrillar arrays.

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